Burner system including a distal flame holder and a non-reactive fluid source

ABSTRACT

A burner includes a distal flame holder, first and second fuel nozzles, a fuel and oxidant source, and a mixing tube disposed upstream from the distal flame holder. Fuel emitted from the first fuel nozzle mixes with oxidant from the oxidant source to form a fuel and oxidant mixture to support combustion in the distal flame holder. A non-reactive fluid source such as recirculated flue gas provides a non-reactive fluid for dilution of the fuel and oxidant mixture to prevent flashback.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority benefit from co-pending U.S. Provisional Patent Application No. 62/798,934, entitled “BURNER SYSTEM INCLUDING A PERFORATED FLAME HOLDER AND A NON-REACTIVE FLUID SOURCE,” filed Jan. 30, 2019 (docket number 2651-232-02); which application, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

SUMMARY

In some embodiments, a burner system may include a distal flame holder, a fuel supply, an oxidant source, and/or a non-reactive fluid source. The distal flame holder may be disposed in a combustion volume and configured to receive and ignite a fuel and oxidant mixture. The fuel supply may be configured to contribute a fuel to the fuel and oxidant mixture. The oxidant source may be configured to contribute an oxidant to the fuel and oxidant mixture. The non-reactive fluid source is configured to deliver a non-reactive fluid in a dilution distance between the distal flame holder and the non-reactive fluid source.

In some embodiments, a burner system may include a distal flame holder, an oxidant conduit, a first fuel nozzle, and/or a non-reactive fluid source. The distal flame holder is configured to hold a combustion reaction of a fuel and an oxidant. The oxidant conduit is configured to direct the oxidant toward the distal flame holder. The first fuel nozzle is oriented to direct a first flow of the fuel into a combustion volume for mixture with the oxidant in a dilution region between the first fuel nozzle and the distal flame holder when a temperature of the distal flame holder is above a predetermined temperature. The non-reactive fluid source is oriented to emit a non-reactive fluid into the dilution region when the distal flame holder is at an operating temperature.

In some embodiments, a method for inhibiting flashback in a burner system may include supplying an oxidant to a combustion volume, directing a fuel via a first fuel nozzle to a dilution region of the combustion volume between the first fuel nozzle and a distal flame holder, mixing the oxidant with the fuel from the first fuel nozzle in the dilution region to provide a mixture of the fuel and the oxidant, and/or providing a non-reactive fluid to the mixture of the fuel and the oxidant in a portion of the dilution region proximate the distal flame holder.

In some embodiments, a multi-stage burner system may include a fuel and oxidant source, a distal flame holder, and/or at least one intermediate flame holder. The oxidant source is configured to emit fuel and oxidant into a combustion volume. The distal flame holder is oriented to receive and ignite a first mixture of the fuel and the oxidant downstream of the fuel and oxidant source. The at least one intermediate flame holder is disposed between the fuel and oxidant source and the distal flame holder and oriented to receive a second mixture of the fuel and the oxidant.

In some embodiments, a method of utilizing a multi-stage burner system may include directing an oxidant into a combustion volume, directing a fuel via a first fuel nozzle toward an intermediate flame holder disposed proximate the first fuel nozzle in a dilution region between the first fuel nozzle and a distal flame holder, and/or holding a flame at the intermediate flame holder supported by a mixture of the oxidant and the fuel from the first fuel nozzle. When the distal flame holder is at a predetermined temperature, a second fuel nozzle may direct the fuel toward the distal flame holder via the dilution region, mix the oxidant and the fuel from the second fuel nozzle to provide a second mixture of the fuel and the oxidant for combustion at the distal flame holder, dilute the second mixture of the fuel and the oxidant with combustion products of the flame within the dilution region and burn the second mixture of the fuel and the oxidant substantially at the distal flame holder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a burner system, according to an embodiment.

FIG. 2 is a simplified perspective view of a burner system including a distal flame holder, according to an embodiment.

FIG. 3 is a side sectional diagram of a portion of the distal flame holders of FIGS. 1 and 2, according to embodiments.

FIG. 4 is a flow chart showing a method for operating a burner system including the distal flame holder of FIGS. 2 and 3, according to embodiments.

FIG. 5 illustrates a flashback phenomenon that may occur in a burner system, according to an embodiment.

FIG. 6 is a block diagram of a burner system, according to an embodiment.

FIG. 7 is a block diagram of a burner system, according to an embodiment.

FIGS. 8A-8B are flow charts showing a method for operating a burner system including the distal flame holder according to FIGS. 6 and 7, according to embodiments.

FIG. 9 is a block diagram of a multi-stage burner system, according to an embodiment.

FIGS. 10A-10B are flow charts showing a method for operating a multi-stage burner system including an intermediate flame holder and a distal flame holder, according to an embodiment.

FIG. 11A is a simplified perspective view of a combustion system, including another alternative distal flame holder, according to an embodiment.

FIG. 11B is a simplified side sectional diagram of a portion of the reticulated ceramic distal flame holder of FIG. 11A, according to an embodiment.

FIG. 12 is an illustration of a burner system, according to an embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise.

The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.

Certain burner systems utilize a distal flame holder disposed downstream from a fuel and oxidant source. Fuel and oxidant in sufficiently flammable proportion enters perforations at an input side of the distal flame holder, is ignited and burned therein. Ideally the resulting combustion occurs at a speed and temperature that minimizes undesirable combustion products such as nitrogen oxides (NOx) while providing sufficient thermal energy to the distal flame holder to sustain combustion of the continuously received fuel and oxidant mixture and to provide heat for the relevant burner application.

The inventors have observed, in a variety of furnace applications, undesirable combustion oscillations occurring between the distal flame holder and the fuel and oxidant source. (Although not necessarily restricted to a confined furnace configuration—e.g., a water heater, boiler, or once-through steam generator (OTSG)—such applications are representative environments that can permit such combustion oscillations.)

When fuel and oxidant are in sufficiently combustible proportion and exposed to sufficient heat for ignition, they can undesirably ignite upstream of the distal flame holder. This phenomenon tends to oscillate and is referred to herein as “flashback,” and is sometimes colloquially referred to as “huffing.” In some implementations, insufficiently and/or non-uniformly cooled oxidant, e.g., flue gas, can be recirculated from downstream of the distal flame holder, resulting in a fuel-oxidant mixture with a sufficiently high temperature that the mixture may ignite prior to reaching the distal flame holder. The flashback reduces the efficiency of the burner at least in part because heat from this premature combustion is not (in a gas-fired burner) radiant heat, is not sufficiently absorbed by the distal flame holder, and is thus wasted. Combustion products from the flashback can dilute the mixture and thus temporarily snuff the flashback combustion. Hence the oscillating nature of flashback.

As described below with respect to FIG. 3, the distal flame holder 102 may be structured to absorb energy from a combustion reaction 302 substantially contained within a perforation 210 of the distal flame holder 102. The distal flame holder 102 can emit that energy as radiant heat, something a gas-fueled flame alone does poorly due to its low emissivity. Thus, when a gas-fueled combustion reaction 302 occurs outside the distal flame holder 102, as during flashback, heat is wasted. Moreover, such combustion reaction 302 may be incomplete, resulting in undesired combustion products which may result in unacceptable emissions levels (e.g., NOx) that may adversely affect downstream structures such as the distal flame holder 102 due to fluctuations in temperature, and may present sonic disturbances.

It is acknowledged that the term “flashback” is often used to refer to systems in which pre-mixed fuel and oxidant are supplied directly and a premature ignition can result in a flame traveling into the premix supply. In the present disclosure, flashback can also be used in reference to non-premix applications, where fuel (e.g., 106 b, 506 b) and oxidant (e.g., 106 a, 506 a) become mixed (e.g., 206, 506) in a dilution region (e.g., 510) between the fuel and oxidant source(s) (e.g., 202) and the distal flame holder 102, such as in a space upstream of the distal flame holder 102, as discussed below with relation to FIG. 5.

Disclosed herein are mechanisms and methods for preventing flashback, including structures and processes for delay of sufficient fuel and oxidant mixing, and reduction of fuel and oxidant mixture temperature in a dilution region. The burner configurations disclosed in detail below are directed to reduce or eliminate such flashback by reducing the possibility of the fuel and oxidant mixture igniting prior to reaching the distal flame holder 102. According to an interpretation, disclosed embodiments dilute the fuel and oxidant mixture to prevent ignition upstream of the distal flame holder 102. According to another interpretation, disclosed embodiments create a hot temperature region upstream of the distal flame holder 102, which hot temperature region alters oxidant circulation/recirculation patterns and thus prevents premature ignition of the fuel and oxidant mixture.

The fuel and oxidant mixture may be diluted by introducing a non-reactive or non-combustible fluid (e.g., gas) upstream of the distal flame holder 102. As used herein, the term “non-reactive fluid” may include fluids that are reactive under certain conditions but are non-reactive in conditions relevant to the embodiment. Likewise, the term “non-combustible fluid” as used herein may include fluids that are not combustible in conditions relevant to the disclosed embodiment(s), but could be combustible under other conditions. Naturally, fluids, such as certain inert or noble gases, that are not reactive under any conditions are also contemplated by the inventors as being non-reactive and/or non-combustible.

A non-reactive or non-combustible fluid may include, for example, a dedicated non-reactive gas, or may include combustion products from burning a portion of fuel upstream of the distal flame holder 102. In an implementation utilizing combustion products as a non-reactive fluid, a dedicated first-stage flame holder may be utilized to hold a flame. Alternatively, a fuel nozzle may itself act as a first-stage flame holder sufficient to support such combustion, e.g., in a flame-stabilized implementation. Use of two combustion regions (first-stage combustion at/near the first-stage flame holder and second-stage combustion at the distal flame holder 102) may permit a low NOx burner to achieve even lower emissions by utilizing the distal flame holder 102 for combustion while ensuring combustion temperatures sufficiently low to avoid creation of NOx.

These and other embodiments are described in detail below with reference to the drawings. FIG. 1 is a block diagram of a burner system 100, according to an embodiment. The burner system 100 includes a distal flame holder 102, a fuel and oxidant source 202, and a mixing tube 110. The fuel and oxidant source 202 may include an oxidant conduit 104 for delivery of an oxidant 106 a, and one or more fuel nozzle(s) 118 for main delivery of a fuel 106 b. The fuel 106 a and the oxidant 106 b mix in the mixing tube 110 en route to the distal flame holder 102, creating a fuel and oxidant mixture 206. The distal flame holder 102 is disposed and oriented to receive and (when at an operating temperature) to ignite the fuel and oxidant mixture 206. The oxidant conduit 104 provides a pathway for the oxidant 106 a (e.g., air), and directs the oxidant 106 a toward the distal flame holder 102. The fuel nozzle(s) 118 direct the fuel 106 b toward the distal flame holder 102. The fuel nozzle(s) 118 may receive the fuel 106 b from a fuel reservoir or pipeline (not shown, each or both referred to herein as a fuel supply) via a fuel supply line 108. The burner system 100 may include a single fuel nozzle 118 or a plurality of the fuel nozzle(s) 118, each disposed and configured as described herein. The fuel 106 b emitted by the fuel nozzle(s) 118, and the oxidant 106 a emitted by the oxidant conduit 104 become mixed as they travel toward the distal flame holder 102. The fuel 106 b and the oxidant 106 a achieve a sufficiently uniform fuel and oxidant mixture (see element 206 in FIG. 2) to permit efficient and uniform combustion within the distal flame holder 102 at the operating temperature.

The burner system 100 may include one or more second nozzle(s) 112. The one or more second nozzle(s) 112 may be oriented and placed to directly or indirectly supply a pilot flame to the fuel and oxidant mixture 206. In some implementations, the second nozzle(s) 112 may receive fuel from a pilot fuel supply line 114 or may be in fluid connection with the fuel supply line 108. In some implementations, the second nozzle(s) 112 may be disposed at the fuel dump plane proximate to the fuel nozzles(s) 118. Alternatively, the second nozzle(s) 112 may be disposed distally, proximate the distal flame holder 102. Elements of the burner system 100 (and 600, 700, 900, 1100, 1200) are described in greater detail with respect to FIGS. 2-3 and 5-7 below. FIGS. 2-3 illustrate a simplified burner system which is described generally. Those of skill in the art will acknowledge that any or all of the features described below with respect to FIGS. 2-3 may apply to above- and later-described implementations of a burner system, such as the burner systems described in relation to FIG. 1 and FIGS. 5-12.

FIG. 2 is a simplified diagram of a burner system 200 including a distal flame holder (such as distal flame holder 102) configured to hold a combustion reaction, according to an embodiment. It is noted that the distal flame holder 102 may have different configurations. For example, the distal flame holder 102 may be circular, annular, rectangular as illustrated in FIGS. 2-3, or other reasonably fabricated shape. It will be appreciated by those having skill in the art, for example, that other configurations may be implemented, such as oblong, semi-circular, triangular, etc. As used herein, the terms distal flame holder (DFH), distal reaction holder, perforated reaction holder (PFH), perforated reaction holder, porous flame holder, and porous reaction holder shall be considered synonymous unless further definition is provided. Experiments performed by the inventors have shown that burner systems described herein can support very clean combustion. Specifically, in experimental use of the burner systems ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (O₂) concentration with undetectable carbon monoxide (CO) at stack temperatures typical of industrial furnace applications (1400-1600° F.). Moreover, these results did not require any extraordinary measures such as selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), water/steam injection, external or internal flue gas recirculation (FGR), or other extremes that may be required for conventional burners to even approach such clean combustion.

According to embodiments, the burner system 200 includes a fuel and oxidant source 202 disposed to output fuel (e.g., 106 b) and oxidant (e.g., 106 a) into a combustion volume 204 to form a fuel and oxidant mixture 206. As used herein, the terms combustion volume, combustion chamber, furnace volume, and the like shall be considered synonymous unless further definition is provided. The distal flame holder 102 is disposed in the combustion volume 204 and positioned to receive the fuel and oxidant mixture 206. FIG. 3 is side sectional diagram 300 of a portion of a type of distal flame holder 102 of FIG. 2 that includes a perforated reaction holder, according to an embodiment. Referring to FIGS. 2 and 3, the distal flame holder 102 includes a distal flame holder body 208 defining a plurality of perforations 210 aligned to receive the fuel and oxidant mixture 206 from the fuel and oxidant source 202. As used herein, the terms perforation, pore, aperture, elongated aperture, and the like, in the context of the distal flame holder 102, shall be considered synonymous unless further definition is provided. The perforations 210 are configured to collectively hold a combustion reaction 302 supported by the fuel and oxidant mixture 206.

The fuel (106 b) can include hydrogen, a hydrocarbon gas, a vaporized hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel (106 b) can be a single species or can include a mixture of gas(es), vapor(s), atomized liquid(s), and/or pulverized solid(s). For example, in a process heater application, the fuel (106 b) can include fuel gas or byproducts from the process that include carbon monoxide (CO), hydrogen (H₂), and methane (CH₄). In another application, the fuel (106 b) can include natural gas (mostly CH₄) or propane (C₃H₈). In another application, the fuel (106 b) can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant (106 a) can include oxygen carried by air and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein.

According to an embodiment, the distal flame holder body 208 can be bounded by an input face 212 disposed to receive the fuel and oxidant mixture 206, an output face 214 facing away from the fuel and oxidant source 202, and a peripheral surface 216 defining a lateral extent of the distal flame holder 102. The plurality of perforations 210 which are defined by the distal flame holder body 208 extend from the input face 212 to the output face 214. The plurality of perforations 210 can receive the fuel and oxidant mixture 206 at the input face 212. The fuel and oxidant mixture 206 can then combust in or near the plurality of perforations 210 and combustion products can exit the plurality of perforations 210 at or near the output face 214.

According to an embodiment, such a perforated flame holder 102 is configured to hold a majority of the combustion reaction 302 within the perforations 210. For example, on a steady-state basis, more than half the molecules of fuel output into the combustion volume 204 by the fuel and oxidant source 202 may be converted to combustion products between the input face 212 and the output face 214 of the perforated flame holder 102. According to an alternative interpretation, more than half of the heat output by the combustion reaction 302 may be output between the input face 212 and the output face 214 of the perforated flame holder 102. Under nominal operating conditions, the perforations 210 can be configured to collectively hold at least 80% of the combustion reaction 302 between the input face 212 and the output face 214 of the distal flame holder 102. In some experiments, the inventors produced a combustion reaction 302 that was apparently wholly contained in the perforations 210 between the input face 212 and the output face 214 of the distal flame holder 102. According to an alternative interpretation, the distal flame holder 102 can support combustion between the input face 212 and the output face 214 when combustion is “time-averaged.” For example, during transients, such as before the distal flame holder 102 is fully heated, or if too high a (cooling) load is placed on the system, the combustion may travel somewhat downstream from the output face 214 of the distal flame holder 102.

While a “flame” is described in a manner intended for ease of description, it should be understood that in some instances, no visible flame is present. Combustion occurs primarily within the perforations 210, but the “glow” of combustion heat is dominated by a visible glow of the distal flame holder 102 itself. In instances, the inventors have observed combustion oscillations (referred to herein as “flashback” and informally as “huffing”) in a variety of burner applications. Flashback includes momentary ignition of fuel (106 b) and oxidant (106 a) in a region lying between the input face 212 of the distal flame holder 102 and a fuel nozzle 218 (described below), within the dilution distance DD. Such transient flashback is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations 210 of the distal flame holder 102, between the input face 212 and the output face 214. In still other instances, the inventors have noted apparent combustion occurring above the output face 214 of the distal flame holder 102, but still a majority of combustion occurred within the distal flame holder 102 as evidenced by the continued visible glow (a visible wavelength tail of blackbody radiation) from the distal flame holder 102. A perforated flame holder 102 can be configured to receive heat from the combustion reaction 302 and output a portion of the received heat as thermal radiation 304 to heat-receiving structures (e.g., furnace walls and/or radiant section working fluid tubes) in or adjacent to the combustion volume 204. As used herein, terms such as thermal radiation, infrared radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody radiation of electromagnetic energy, primarily in infrared wavelengths.

Referring especially to FIG. 3, the distal flame holder 102 outputs another portion of the received heat to the fuel and oxidant mixture 206 received at the input face 212 of the distal flame holder 102. The distal flame holder body 208 may receive heat from the (exothermic) combustion reaction 302 at least in heat receiving regions 306 of perforation walls 308. Experimental evidence has suggested to the inventors that the position of the heat receiving regions 306, or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls 308. In some experiments, the location of maximum receipt of heat was apparently between ⅓ and ½ of the distance from the input face 212 to the output face 214 (i.e., somewhat nearer to the input face 212 than to the output face 214). The inventors contemplate that the heat receiving regions 306 may lie nearer to the output face 214 of the distal flame holder 102 under other conditions. According to an interpretation there is no clearly defined edge of the heat receiving regions 306 (or, for that matter, of heat output regions 310, described below). For ease of understanding, the heat receiving regions 306 and the heat output regions 310 will be described as particular regions 306, 310.

The distal flame holder body 208 can be characterized by a heat capacity. The distal flame holder body 208 may hold heat from the combustion reaction 302 in an amount corresponding to the heat capacity multiplied by temperature rise, and transfer the heat from the heat receiving regions 306 to the heat output regions 310 of the perforation walls 308. Generally, the heat output regions 310 are nearer to the input face 212 than are the heat receiving regions 306. According to one interpretation, the distal flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via thermal radiation, depicted graphically as 304. According to another interpretation, the distal flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via heat conduction along heat conduction paths 312. The inventors contemplate that both radiation and conduction heat transfer mechanisms may be operative in transferring heat from the heat receiving regions 306 to the heat output regions 310. In this way, the distal flame holder 102 may act as a heat source to maintain the combustion reaction 302, even under conditions where the combustion reaction 302 would not be stable when supported from a conventional flame holder.

The inventors believe that the distal flame holder 102 causes the combustion reaction 302 to occur within thermal boundary layers 314 formed adjacent to the walls 308 of the perforations 210. As the relatively cool fuel and oxidant mixture 206 approaches the input face 212, the flow is split into portions that respectively travel through individual perforations 210. The hot distal flame holder body 208 transfers heat to the fluid, notably within the thermal boundary layers 314 that progressively thicken as more and more heat is transferred to the incoming fuel and oxidant mixture 206. After reaching a combustion temperature (e.g., the auto-ignition temperature of the fuel (106 b)), the reactants continue to flow while a chemical ignition delay time elapses, over which time the combustion reaction 302 occurs. Accordingly, the combustion reaction 302 is shown as occurring within the thermal boundary layers 314. As flow progresses, the thermal boundary layers 314 merge at a merger point 316. The merger point 316 may lie between the input face 212 and the output face 214 that defines the ends of the perforations 210. At some point, the combustion reaction 302 causes the flowing gas (and plasma) to output more heat to the distal flame holder body 208 than it receives from the distal flame holder body 208. The heat is received at the heat receiving region 306, is held by the distal flame holder body 208, and is transported to the heat output region 310 nearer to the input face 212, where the heat recycles into the cool reactants (and any included diluent) to raise them to the combustion temperature.

In an embodiment, the perforations 210 are each characterized by a length L defined as a reaction fluid propagation path length between the input face 212 and the output face 214 of the distal flame holder 102. The reaction fluid includes the fuel and oxidant mixture 206 (optionally including nitrogen, flue gas, and/or other “non-reactive” species), reaction intermediates (including transition states in a plasma that characterizes the combustion reaction 302), and reaction products.

The perforations 210 can be each characterized by a transverse dimension D between opposing perforation walls 308. The inventors have found that stable combustion can be maintained in the distal flame holder 102 if the length L of each perforation 210 is at least four times the transverse dimension D of the perforation 210. In other embodiments, the length L can be greater than six times the transverse dimension D. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D. Preferably, the length L is sufficiently long for thermal boundary layers 314 formed adjacent to the perforation walls 308 in a reaction fluid flowing through the perforations 210 to converge at merger points 316 within the perforations 210 between the input face 212 and the output face 214 of the distal flame holder 102. In experiments, the inventors have found L/D ratios between 12 and 48 to work well (i.e., produce low NOx, produce low CO, and maintain stable combustion).

The distal flame holder body 208 can be configured to convey heat between adjacent perforations 210. The heat conveyed between adjacent perforations 210 can be selected to cause heat output from the combustion reaction portion 302 in a first perforation 210 to supply heat to stabilize a combustion reaction portion 302 in an adjacent perforation 210.

Referring especially to FIG. 2, the fuel and oxidant source 202 can further include the fuel nozzle 218, configured to output fuel (e.g., 106 b), and an oxidant source 220 configured to output a fluid including the oxidant (e.g., 106 a). For example, the fuel nozzle 218 can be configured to output pure fuel. The oxidant source 220 can be configured to output combustion air carrying oxygen.

The distal flame holder 102 can be held by a distal flame holder support structure 222 configured to hold the distal flame holder 102 a distance DD away from the fuel nozzle 218. The fuel nozzle 218 can be configured to emit a fuel jet selected to entrain the oxidant (106 a) to form the fuel and oxidant mixture 206 as the fuel jet and oxidant (106 a) travel along a path to the distal flame holder 102 through a dilution distance DD between the fuel nozzle 218 and the distal flame holder 102. Additionally or alternatively (particularly when a blower is used to deliver oxidant combustion air (106 a)), the oxidant or combustion air source 220 can be configured to entrain the fuel (106 b), and the fuel (106 b) and oxidant (106 a) travel through the dilution distance DD. In some embodiments, a flue gas recirculation path 224 can be provided. Additionally or alternatively, the fuel nozzle 218 can be configured to emit a fuel jet selected to entrain the oxidant (106 a) and to entrain flue gas as the fuel jet travels through a dilution distance DD between the fuel nozzle 218 and the input face 212 of the distal flame holder 102.

The fuel nozzle 218 can be configured to emit the fuel (106 b) through one or more fuel orifices 226 having a dimension that is referred to as “nozzle diameter.” The distal flame holder support structure 222 can support the distal flame holder 102 to receive the fuel and oxidant mixture 206 at a distance DD away from the fuel nozzle 218 greater than 20 times the nozzle diameter. In another embodiment, the distal flame holder 102 is disposed to receive the fuel and oxidant mixture 206 at a distance DD away from the fuel nozzle 218 between 100 times and 1100 times the nozzle diameter. Preferably, the distal flame holder support structure 222 is configured to hold the distal flame holder 102 about 200 times the nozzle diameter or more away from the fuel nozzle 218.

When the fuel and oxidant mixture 206 travels about 200 times the nozzle diameter or more, the fuel and oxidant mixture 206 is sufficiently homogenized to cause the combustion reaction 302 to output minimal NOx.

The fuel and oxidant source 202 can alternatively include a premix fuel and oxidant source, according to an embodiment. A premix fuel and oxidant source (not shown) can include a premix chamber, a fuel nozzle configured to output fuel into the premix chamber, and an air channel configured to output combustion air into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the distal flame holder 102 and be configured to prevent flame flashback into the premix fuel and oxidant source.

The combustion air source, whether configured for entrainment in the combustion volume 204 or for premixing, can include a blower configured to force air through the fuel and oxidant source 202.

The distal flame holder support structure 222 can be configured to support the distal flame holder 102 from a floor or wall (not shown) of the combustion volume 204, for example. In another embodiment, the distal flame holder support structure 222 supports the distal flame holder 102 from the fuel and oxidant source 202. Alternatively, the distal flame holder support structure 222 can suspend the distal flame holder 102 from an overhead structure (such as a flue, in the case of an up-fired system). The distal flame holder support structure 222 can support the distal flame holder 102 in various orientations and directions.

The distal flame holder 102 can include a single distal flame holder body 208. In another embodiment, the distal flame holder 102 can include a plurality of adjacent distal flame holder sections (not shown) that collectively provide a tiled distal flame holder 102. The distal flame holder support structure 222 can be configured to support the plurality of distal flame holder sections. The distal flame holder support structure 222 can include a metal superalloy, a cementatious, and/or ceramic refractory material. In an embodiment, the plurality of adjacent distal flame holder sections can be joined with a fiber reinforced refractory cement. The distal flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least twice a thickness dimension T between the input face 212 and the output face 214. In another embodiment, the distal flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least three times, at least six times, or at least nine times a thickness dimension T between the input face 212 and the output face 214 of the distal flame holder 102.

In an embodiment, the distal flame holder 102 can have a width dimension W less than a width of the combustion volume 204. This can allow the flue gas recirculation path 224 from above to below the distal flame holder 102 to lie between the peripheral surface 216 of the distal flame holder 102 and the combustion volume wall (not shown).

Referring again to both FIGS. 2 and 3, the perforations 210 can include elongated squares, each of the elongated squares has a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations 210 can include elongated hexagons, each of the elongated hexagons has a transverse dimension D between opposing sides of the hexagons. In another embodiment, the perforations 210 can include hollow cylinders, where each of the hollow cylinders has a transverse dimension D corresponding to a diameter of the cylinders. In another embodiment, the perforations 210 can each be a frustrum of a cone, each having a transverse dimension D that is rotationally symmetrical about a length axis that extends from the input face 212 to the output face 214. The perforations 210 can each have a transverse dimension D equal to or greater than a quenching distance of the fuel based on standard reference conditions.

In one range of embodiments, each of the plurality of perforations 210 has a lateral dimension D between 0.05 inch and 1.0 inch. Preferably, each of the plurality of perforations 210 has a lateral dimension D between 0.1 inch and 0.5 inch. For example, the perforations 210 can each have a lateral dimension D of about 0.2 to 0.4 inch.

The void fraction of a distal flame holder 102 is defined as the total volume of all perforations 210 in a section of the distal flame holder 102 divided by a total volume of the distal flame holder 102 including distal flame holder body 208 and the perforations 210. The distal flame holder 102 should have a void fraction between 0.10 and 0.90. In an embodiment, the distal flame holder 102 can have a void fraction between 0.30 and 0.80. In another embodiment, the distal flame holder 102 can have a void fraction of about 0.70. Using a void fraction of about 0.70 was found to be especially effective for producing very low NOx.

The distal flame holder 102 can be formed from a fiber reinforced cast refractory material and/or a refractory material such as an aluminum silicate material. For example, the distal flame holder 102 can be formed from mullite or cordierite. Additionally or alternatively, the distal flame holder body 208 can include a metal superalloy such as Inconel or Hastelloy. The distal flame holder body 208 can define a honeycomb.

The inventors have found that the distal flame holder 102 can be formed from VERSAGRID ® ceramic honeycomb, available from Applied Ceramics, Inc. of Doraville, S.C.

The perforations 210 can be parallel to one another and normal to the input and the output faces 212, 214. In another embodiment, the perforations 210 can be parallel to one another and formed at an angle relative to the input and the output faces 212, 214. In another embodiment, the perforations 210 can be non-parallel to one another. In another embodiment, the perforations 210 can be non-parallel to one another and non-intersecting. In another embodiment, the perforations 210 can be intersecting. The distal flame holder body 208 can be one piece or can be formed from a plurality of sections.

In another embodiment, which is not necessarily preferred, the distal flame holder 102 may be formed from reticulated fibers formed from an extruded ceramic material. The term “reticulated fibers” refers to a netlike structure.

In another embodiment, the distal flame holder 102 can include a plurality of tubes or pipes bundled together. The plurality of perforations 210 can include hollow cylinders and can optionally also include interstitial spaces between the bundled tubes. In an embodiment, the plurality of tubes can include ceramic tubes. Refractory cement can be included between the tubes and configured to adhere the tubes together. In another embodiment, the plurality of tubes can include metal (e.g., superalloy) tubes. The plurality of tubes can be held together by a metal tension member circumferential to the plurality of tubes and arranged to hold the plurality of tubes together. The metal tension member can include stainless steel, a superalloy metal wire, and/or a superalloy metal band.

The distal flame holder body 208 can alternatively include stacked perforated sheets of material, each sheet having openings that connect with openings of subjacent and superjacent sheets. The perforated sheets can include perforated metal sheets, ceramic sheets and/or expanded sheets. In another embodiment, the distal flame holder body 208 can include discontinuous packing bodies such that the perforations 210 are formed in the interstitial spaces between the discontinuous packing bodies. In one example, the discontinuous packing bodies include structured packing shapes. In another example, the discontinuous packing bodies include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig ring, ceramic Berl saddles, ceramic Intalox saddles, and/or metal rings or other shapes (e.g., Super Raschig Rings) that may be held together by a metal cage.

The inventors contemplate various explanations for why burner systems including the distal flame holder 102 provide such clean combustion. In one aspect, the distal flame holder 102 acts as a heat source to maintain a combustion reaction 302 even under conditions where a combustion reaction would not be stable when supported by a conventional flame holder. This capability can be leveraged to support combustion using a leaner fuel-to-oxidant mixture than is typically feasible. Thus, according to an embodiment, at the point where the fuel and oxidant mixture 206 contacts the input face 212 of the distal flame holder 102, an average fuel-to-oxidant ratio of the fuel and oxidant mixture 206 is below a (conventional) lower combustion limit of the fuel component of the fuel and oxidant mixture 206—lower combustion limit defines the lowest concentration of fuel at which a fuel and air mixture will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.).

According to one interpretation, the fuel and oxidant mixtures 206 supported by the distal flame holder 102 may be more fuel-lean than mixtures that would provide stable combustion in a conventional burner. Combustion near a lower combustion limit of fuel generally burn at a lower adiabatic flame temperature than mixtures near the center of the lean-to-rich combustion limit range. Lower flame temperatures generally evolve a lower concentration of oxides of nitrogen (NOx) than higher flame temperatures. In conventional flames, too-lean combustion is generally associated with high CO concentration at the stack. In contrast, the distal flame holder 102 and systems including the distal flame holder 102 described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. In some embodiments, the inventors achieved stable combustion at what was understood to be very lean mixtures (that nevertheless produced only about 3% or lower measured O₂ concentration at the stack). Moreover, the inventors believe the perforation walls 308 may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperature.

According to another interpretation, production of NOx can be reduced if the combustion reaction 302 occurs over a very short duration of time. Rapid combustion causes the reactants (including oxygen and entrained nitrogen) to be exposed to NOx-formation temperature for a time too short for NOx formation kinetics to cause significant production of NOx. The time required for the reactants to pass through the distal flame holder 102 is very short compared to a conventional flame. The low NOx production associated with distal flame holder 102 combustion may thus be related to the short duration of time required for the reactants (and entrained nitrogen) to pass through the distal flame holder 102.

Since CO oxidation is a relatively slow reaction, the time for passage through the distal flame holder 102 (perhaps plus time passing toward the flue from the distal flame holder 102) is apparently sufficient and at sufficiently elevated temperature, in view of the very low measured (experimental and full scale) CO concentrations, for oxidation of CO to carbon dioxide (CO₂).

FIG. 4 is a flow chart showing a method 400 for operating a burner system including the distal flame holder (e.g., 102) shown and described herein. To operate a burner system including a distal flame holder, the distal flame holder is first heated to a temperature sufficient to maintain combustion of the fuel and oxidant mixture (referred to herein as an operating temperature).

According to a simplified description, the method 400 begins with step 402, wherein the distal flame holder is preheated to a start-up temperature, T_(S). After the distal flame holder is raised to the start-up temperature, the method proceeds to step 404, wherein fuel and oxidant are provided to the distal flame holder and combustion is held by thereby.

According to a more detailed description, step 402 begins with step 406, wherein start-up energy is provided at the distal flame holder. Simultaneously or following providing start-up energy, a decision step 408 determines whether the temperature T of the distal flame holder is at or above the start-up temperature, T_(S). As long as the temperature of the distal flame holder is below its start-up temperature, the method loops between steps 406 and 408 within the preheat step 402. In decision step 408, if the temperature T of at least a predetermined portion of the distal flame holder is greater than or equal to the start-up temperature, the method 400 proceeds to overall step 404, wherein fuel and oxidant is supplied to and combustion is held by the distal flame holder.

Step 404 may be broken down into several discrete steps, at least some of which may occur simultaneously.

Proceeding from decision step 408, a fuel and oxidant mixture is provided to the distal flame holder, as shown in step 410. The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and combustion air source, for example. In this approach, the fuel and combustion air are output in one or more directions selected to cause the fuel and combustion air mixture to be received by an input face of the distal flame holder. The fuel may entrain the combustion air (or alternatively, the combustion air may dilute the fuel) to provide a fuel and oxidant mixture at the input face of the distal flame holder at a fuel dilution selected for a stable combustion reaction that can be held within the perforations of the distal flame holder.

Proceeding to step 412, the combustion reaction is held by the distal flame holder. In step 414, heat may be output from the distal flame holder. The heat output from the distal flame holder may be used to power an industrial process, heat a working fluid, generate electricity, or provide motive power, for example.

In optional step 416, the presence of combustion may be sensed. Various sensing approaches have been used and are contemplated by the inventors. Generally, combustion held by the distal flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, and/or other known combustion sensing apparatuses. In an additional or alternative variant of step 416, a pilot flame or other ignition source may be provided to cause ignition of the fuel and oxidant mixture in the event combustion is lost at the distal flame holder.

Proceeding to decision step 418, if combustion is sensed not to be stable, the method 400 may exit to step 424, wherein an error procedure is executed. For example, the error procedure may include turning off fuel flow, re-executing the preheating step 402, outputting an alarm signal, igniting a stand-by combustion system, or other steps. If, in decision step 418, combustion in the distal flame holder is determined to be stable, the method 400 proceeds to decision step 420, wherein it is determined if combustion parameters should be changed. If no combustion parameters are to be changed, the method loops (within step 404) back to step 410, and the combustion process continues. If a change in combustion parameters is indicated, the method 400 proceeds to step 422, wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step 404) back to step 410, and combustion continues.

Combustion parameters may be scheduled to be changed, for example, if a change in heat demand is encountered. For example, if less heat is required (e.g., due to decreased electricity demand, decreased motive power requirement, or lower industrial process throughput), the fuel and oxidant flow rate may be decreased in step 422. Conversely, if heat demand is increased, then fuel and oxidant flow may be increased. Additionally or alternatively, if the combustion system is in a start-up mode, then fuel and oxidant flow may be gradually increased to the distal flame holder over one or more iterations of the loop within step 404.

Referring again to FIG. 2, the burner system 200 may include a heater 228 operatively coupled to the distal flame holder 102. As described in conjunction with FIGS. 3 and 4, the distal flame holder 102 operates by outputting heat to the incoming fuel and oxidant mixture 206. After combustion is established, this heat is provided by the combustion reaction 302; but before combustion is established, the heat is provided by the heater 228.

Various heating apparatuses have been used and are contemplated by the inventors. In some embodiments, the heater 228 can include a flame holder configured to support a flame disposed to heat the distal flame holder 102. The fuel and oxidant source 202 can include a fuel nozzle 218 configured to emit a fuel stream, and an oxidant source 220 configured to output combustion air (oxidant) adjacent to the fuel stream. The fuel nozzle 218 and air source 220 can be configured to output the fuel stream to be progressively diluted by the combustion air. The distal flame holder 102 can be disposed to receive a diluted fuel and air mixture 206 that supports a combustion reaction 302 that is stabilized by the distal flame holder 102 when the distal flame holder 102 is at an operating temperature. A start-up flame holder, in contrast, can be configured to support a start-up flame at a location corresponding to a relatively rich fuel and air mixture that is stable without stabilization provided by the heated distal flame holder 102.

The burner system 200 can further include a controller 230 operatively coupled to the heater 228 and to a data interface 232. For example, the controller 230 can be configured to control a start-up flame holder actuator configured to cause the start-up flame holder to hold the start-up flame when the distal flame holder 102 needs to be pre-heated and to not hold the start-up flame when the distal flame holder 102 is at an operating temperature (e.g., when T T_(S)).

Various approaches for actuating a start-up flame are contemplated. In one embodiment, the start-up flame holder includes a mechanically-actuated bluff body configured to be actuated to intercept the fuel and oxidant mixture 206 to cause heat-recycling vortices and thereby hold a start-up flame; or to be actuated to not intercept the fuel and oxidant mixture 206 to cause the fuel and oxidant mixture 206 to proceed to the distal flame holder 102. In another embodiment, a fuel control valve, blower, and/or damper may be used to select a fuel and oxidant mixture flow rate that is sufficiently low for a start-up flame to be jet-stabilized; and upon reaching a distal flame holder 102 operating temperature, the flow rate may be increased to “blow out” the start-up flame. In another embodiment, the heater 228 may include an electrical power supply operatively coupled to the controller 230 and configured to apply an electrical charge or voltage to the fuel and oxidant mixture 206. An electrically conductive start-up flame holder may be selectively coupled to a voltage ground or other voltage selected to attract the electrical charge in the fuel and oxidant mixture 206. The attraction of the electrical charge was found by the inventors to cause a start-up flame to be held by the electrically conductive start-up flame holder.

In another embodiment, the heater 228 may include an electrical resistance heater configured to output heat to the distal flame holder 102 and/or to the fuel and oxidant mixture 206. The electrical resistance heater 228 can be configured to heat up the distal flame holder 102 to an operating temperature.

The heater 228 can further include a power supply and a switch operable, under control of the controller 230, to selectively couple the power supply to the electrical resistance heater 228.

An electrical resistance heater 228 can be formed in various ways. For example, the electrical resistance heater 228 can be formed from KANTHAL® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstahammar, Sweden) threaded through at least a portion of the perforations 210 defined formed by the distal flame holder body 208. Alternatively, the heater 228 can include an inductive heater, a high energy (e.g. microwave or laser) beam heater, a frictional heater, or other types of heating technologies.

Other forms of start-up apparatuses are contemplated. For example, the heater 228 can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the air and fuel. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite a fuel and oxidant mixture 206 that would otherwise enter the distal flame holder 102. An electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller 230, which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the fuel and oxidant mixture 206 in or upstream from the distal flame holder 102 before the distal flame holder 102 is heated sufficiently to maintain combustion.

The burner system 200 can further include a sensor 234 operatively coupled to the control circuit 230. The sensor 234 can include a heat sensor configured to detect infrared radiation or a temperature of the distal flame holder 102. The control circuit 230 can be configured to control the heating apparatus 228 responsive to input from the sensor 234. Optionally, a fuel control valve 236 can be operatively coupled to the controller 230 and configured to control a flow of fuel (e.g., 106 b) to the fuel and oxidant source 202. Additionally or alternatively, an oxidant blower or damper 238 can be operatively coupled to the controller 230 and configured to control flow of the oxidant (or combustion air) (e.g., 106 a).

The sensor 234 can further include a combustion sensor operatively coupled to the control circuit 230, the combustion sensor being configured to detect a temperature, video image, and/or spectral characteristic of a combustion reaction 302 held by the distal flame holder 102. The fuel control valve 236 can be configured to control a flow of fuel (106 b) from a fuel source to the fuel and oxidant source 202. The controller 230 can be configured to control the fuel control valve 236 responsive to input from the combustion sensor 234. The controller 230 can be configured to control the fuel control valve 236 and/or the oxidant blower or damper 238 to control a preheat flame type of heater 228 to heat the distal flame holder 102 to an operating temperature. The controller 230 can similarly control the fuel control valve 236 and/or the oxidant blower or damper 238 to change the fuel and oxidant mixture 206 flow responsive to a heat demand change received as data via the data interface 232.

FIG. 5 illustrates the flashback phenomenon that may occur in a burner system 500 that does not employ the structures and methods disclosed herein for addressing flashback. An oxidant conduit 504 and fuel nozzle(s) 518 respectively emit oxidant 506 a and fuel 506 b which are mixed in a dilution region 510 (corresponding to the dilution distance DD between the fuel nozzles 518 and the distal flame holder 102) to provide a fuel and oxidant mixture 506. The distal flame holder 102 is oriented to receive the fuel and oxidant mixture 506 and is configured to, when at an operating temperature, ignite the fuel and oxidant mixture 506. However, as described above, transient “flashback” may occur wherein the fuel and oxidant mixture 506 momentarily ignites in the dilution region 510 between the distal flame holder 102 and the fuel nozzle(s) 518, producing a visible flame 502. According to one interpretation, the fuel 506 b and the oxidant 506 a become sufficiently well mixed to ignite from heat radiating from the distal flame holder 102 before the mixture 506 reaches the distal flame holder 102. The flame 502 depicted is not necessarily representative of flame shape or relative size in an implementation.

FIG. 6 is a block diagram of a burner system 600, according to an embodiment. The burner system 600 may include a distal flame holder 102 configured to receive and ignite a fuel and oxidant mixture 206. As in FIG. 1, the fuel and oxidant mixture 206 may include an oxidant 106 a (partially omitted from FIG. 6 for clearer presentation of other features) and a fuel 106 b. A fuel source may be configured to contribute the fuel 106 b to the fuel and oxidant mixture 206 when the distal flame holder 102 is at an operating temperature above a predetermined threshold temperature. An oxidant source such as an oxidant conduit 104 that conducts air or another oxidant is configured to contribute the oxidant 106 a to the fuel and oxidant mixture 206. A second fuel nozzle 610 constitute a portion of a non-reactive fluid source 110 to provide a non-reactive fluid (i.e., combustion products) 616 in a dilution region (e.g., 510) corresponding to the dilution distance (DD) between the distal flame holder 102 and second nozzle 610.

More specifically, the non-reactive fluid source 110 may include the second fuel nozzle 610, fuel 614 emitted by the second fuel nozzle 610, and an intermediate flame 618. Substantially non-reactive combustion products 616 from the intermediate flame 618 may constitute at least part of the non-reactive fluid 116 described with respect to FIG. 1. As discussed above, the non-reactive combustion products 616 may dilute the fuel and oxidant mixture 206 such that the fuel and oxidant mixture 206 is insufficiently mixed to auto-ignite in conditions present upstream of the distal flame holder 102. The second fuel nozzle 610 may operate as a flame holder for purposes of holding the intermediate flame 618 at a low fuel flow rate.

It will be understood by those having skill in the art that the embodiment illustrated in FIGS. 5-7 and 9 may additionally include a mixing tube, such as the mixing tube 110 illustrated in FIG. 1.

The fuel source may include a fuel supply (not shown), such as a fuel reservoir or pipeline configured to store the fuel 106 b, a fuel supply line 108, and/or a first fuel nozzle 118 that is in fluid connection with the fuel supply via the fuel supply line 108, and is positioned to deliver the fuel 106 b for the fuel and oxidant mixture 206. In some implementations, the fuel supply line 108 (and in some embodiments a separate fluid supply line 112 as shown in FIG. 1) may each attach to a fuel supply manifold (not shown) that receives fuel 106 b from such fuel reservoir(s).

In some embodiments, the second fuel nozzle 610 may operate as a pilot fuel nozzle, indirectly providing the non-reactive fluid (combustion products 616) as described above.

The burner system 600 may further include a fuel flow control mechanism 620 in fluid connection with the second fuel nozzle 610. The fuel flow control mechanism 620 may include a valve and may be configured to permit a first fuel flow rate to the second fuel nozzle 610 during a startup period. The fuel flow control mechanism 620 may be further configured to permit a second fuel flow rate to the second fuel nozzle 610 after the startup period (e.g., during an operational period when the distal flame holder 102 is at an operating temperature). The burner system 600 may include an ignition source 630 (e.g., an igniter) configured to ignite a fuel 614 and the oxidant 106 a proximate the second fuel nozzle 610 to produce the intermediate flame 618. Specifically, the fuel 614 emitted by the second fuel nozzle(s) 610 may be flame stabilized. Combustion of the fuel 614 and the oxidant 106 a may result in the combustion products 616 as a non-reactive fluid for supply to the fuel and oxidant mixture 206. Such dilution prevents flashback as discussed above. The fuel and oxidant mixture 206 when appropriately diluted by the non-reactive fluid 616 may permit combustion to be substantially limited to the distal flame holder 102, with some amount of the combustion flame 103 visible in certain circumstances.

In some embodiments, the second fuel nozzle 610 may operate to preheat the distal flame holder 102 to a predetermined temperature, for example an operating temperature at which the fuel and oxidant mixture 206 will auto-ignite within the perforations 210 of the distal flame holder 102. The fuel flow control mechanism 620 may thus provide fuel at a first fuel flow rate that permits such preheating, and then change the fuel rate to the second fuel flow rate in order to provide the intermediate flame 618 sufficient to generate combustion products 616 as a non-reactive fluid.

The combustion products 616 from the intermediate flame 618 may constitute at least a portion of the non-reactive fluid 616. In some embodiments, for example, the non-reactive fluid 616 may additionally include other non-reactive elements, such as an inert gas provided from a dedicated inert gas source.

Returning briefly to FIG. 1, in some embodiments, the non-reactive fluid source 110 may include a non-reactive fluid supply (not shown) such as a non-reactive fluid reservoir, and a second nozzle 110 a in fluid connection with the non-reactive fluid supply via a non-reactive fluid supply line 112. The second nozzle 110 a may be positioned to deliver the non-reactive fluid 116 directly to the fuel and oxidant mixture 206.

The non-reactive fluid 116 may be an inert gas at temperatures encountered proximate the distal flame holder 102. Some inert gases include helium and argon. Other inert gases are contemplated by the inventors, including, for example, compounds of elements that, while reactive under certain conditions, are not reactive in conditions encountered upstream of the distal flame holder 102 (e.g., in the dilution region 510) during normal operation. In some implementations, the non-reactive fluid 116 may be an otherwise reactive fluid rendered non-reactive upstream of the distal flame holder 102 by a temperature of the otherwise reactive fluid and/or by the presence of other elements. For example, air, recirculated flue gas or even fuel (e.g., 106 b), if sufficiently cool, may act as a non-reactive fluid 116 to delay auto-ignition of the fuel and oxidant mixture 206 until the mixture reaches the distal flame holder 102.

Turning now to FIG. 7, we see a block diagram of a burner system 700, according to another embodiment. In addition to the structural elements described with relation to FIG. 6, the burner system 700 may include a non-reactive fluid source 110 that includes, in addition to the second fuel nozzle 610, the fuel 614 and flame 718, an intermediate flame holder 702 disposed between the second fuel nozzle 610 and the distal flame holder 102. The intermediate flame holder 702 of the non-reactive fluid source 110 may be configured to hold the intermediate flame 618 supported by the oxidant 106 a and the fuel 614 delivered from the second fuel nozzle 610. The burner system 700 may also include an ignition source 730 configured to ignite the fuel 614 and the oxidant 106 a at the intermediate flame holder 702 to produce the flame 718. In one embodiment, the intermediate flame holder 702 is a flame holding feature disposed to hold a pilot flame 718. The non-reactive fluid source 110 is configured to deliver the non-reactive fluid 616 in a region disposed between the pilot flame 718 (supported by the pilot flame holding feature 702) and an ignitable portion of the fuel and air mixture 206. For example, the fuel and air mixture 206 may be characterized by a surface defining a portion of the fuel and air mixture 206 that is within flammability limits. The non-reactive fluid 616 may provide a buffer between the pilot flame 718 and the surface defining the portion of the fuel and air mixture 206 that is within flammability limits such as to prevent the ignition source 730 from interacting with the fuel and air mixture 206 and causing it to ignite prematurely.

In one embodiment, the non-reactive fluid 616 is a substantially non-reactive product of pilot flame combustion. More specifically, the pilot flame 718 held by the pilot flame holding feature 702 may generate at least a portion of the substantially non-reactive fluid 616 in the form of a fuel depleting combustion product. The components of the substantially non-reactive fluid 616 are, at least partly, selected from the group consisting of nitrogen, carbon dioxide, nitrogen oxide, nitrogen dioxide, sulfur oxide, sulfur dioxide, oxygen, and argon.

In one embodiment, the ignition source 730 in the burner system 700 includes an ignitor configured to selectably ignite the fuel and oxidant mixture 206. The ignitor 730 is disposed adjacent to the pilot flame 718 and the fuel and oxidant mixture 206.

In one embodiment, the non-reactive fluid 616 includes flue gas drawn into the fuel and air mixture 206. For example, the non-reactive fluid 616 may comprise flue gas from downstream of the distal flame holder 102 and combustion reaction products generated by the pilot flame 718. In another embodiment, the burner system (100, 500, 600, 700) may include a flue gas recirculation (FGR) element (e.g., path 224) that may recirculate a portion of combustion products as flue gas resulting from combustion at or in the distal flame holder 102. The FGR further may cause the collected flue gas to be directed (or, recirculated) to an area between the fuel nozzle(s) 118 and the distal flame holder 102 to act, at least in part, as a non-reactive fluid 616. In some embodiments, the recirculated flue gas may include elements that dilute (mix with) the fuel and oxidant mixture. The resulting flue gas-containing fuel and oxidant mixture is then subjected to combustion at the distal flame holder 102. Consequently, pollutants originally found in the flue gas may be reduced by the further combustion thereof. According to an embodiment, the FGR may include a dedicated flue gas conduit (not shown) having a recirculated flue gas output oriented to provide the recirculated flue gas directly to a position between the distal flame holder 102 and the fuel nozzle(s) 118 where the fuel and oxidant mixture 206 is most likely (from observation) to prematurely ignite. For example, the recirculated flue gas output may direct the recirculated flue gas laterally adjacent (i.e., parallel) to the input face 212 of the distal flame holder 102. According to an embodiment, (e.g., burner system 100 in FIG. 1), a mixing tube 110 may be disposed between the distal flame holder 102 and the fuel nozzles 118, and the flue gas recirculation path may direct flue gas at a gap between an end of the mixing tube 110 and a floor of the combustion chamber. Alternatively, the recirculated flue gas output may direct the recirculated flue gas at an angle oblique to an axis of fuel flow from the fuel nozzle(s) 118. According to another embodiment, the recirculated flue gas output may introduce the recirculated flue gas alongside oxidant or combustion air as part of the fuel and oxidant source (202, in FIG. 2).

The burner system 700 may further include the fuel flow control mechanism 620 in fluid connection with the second fuel nozzle 610. As described with respect to FIG. 6, the fuel flow control mechanism 620 may be configured to permit a first fuel flow rate to the second fuel nozzle 610 during a startup period and is configured to permit a second fuel flow rate to the second fuel nozzle 610 after the startup period. The first fuel flow rate of the fuel 614 may be higher than the second fuel flow rate.

The intermediate flame holder 702 may be a bluff body or an electrically conductive flame holder disposed across the fuel path between the second fuel nozzle(s) 610 and the distal flame holder 102. The intermediate flame holder 702 may support a combustion reaction (e.g., intermediate flame 718) fueled by a low-rate (compared to a fuel rate of fuel emitted from the first fuel nozzle(s) 118) fuel flow emitted from second fuel nozzle(s) 610 and oxidant 106 a emitted from the oxidant conduit 104.

In some embodiments, the intermediate flame holder 702 may be disposed a distance from the distal flame holder 102 sufficient, at the first fuel flow rate, to cool the combustion products 616 below an auto-ignition temperature of the fuel and oxidant mixture 206.

Another embodiment is described with reference to FIGS. 1, 6, and 7. A burner system 100, 600, 700 may include a distal flame holder 102 configured to hold a combustion reaction (302) of a fuel 106 b and an oxidant 106 a. An oxidant conduit 104 may direct the oxidant 106 a toward the distal flame holder 102. A first fuel nozzle 118 may be oriented to direct a first flow of the fuel 106 b into a combustion volume (e.g., 204) for mixture with the oxidant 106 a in a dilution region (e.g., 510) between the first fuel nozzle 118 and the distal flame holder 102 when a temperature of the distal flame holder 102 is above a predetermined temperature. A non-reactive fluid source 110 may be oriented to emit a non-reactive fluid 116, 616 into the dilution region 510 when the distal flame holder 102 is at an operating temperature.

In some embodiments, the non-reactive fluid source 110 may include a second fuel nozzle 610 oriented to emit a second flow of the fuel 614 into the combustion volume (e.g., 204) and/or toward an intermediate flame holder 702 disposed between the second fuel nozzle 610 and the distal flame holder 102. The intermediate flame holder 702 may be configured to support a flame 718 in the dilution region 510, the flame 718 produced by combustion of a mixture of the oxidant 106 a and the second flow of the fuel 614 from the second fuel nozzle 610.

In some embodiments, the non-reactive fluid 616 provided by the non-reactive fluid source 110 may include combustion products 616 of the flame 618, 718. The burner system 700 may further include an ignition source 730 disposed proximate the intermediate flame holder 702 and configured to ignite the mixture of the oxidant 106 a and the second flow of the fuel 614.

In some embodiments, such a burner system may further include a first valve 619 configured to control a flow rate of the first flow of the fuel 106 b. In some embodiments, a second valve 620 may be configured to control a flow rate of the second flow of the fuel 614. The valves 619, 620 may be manually adjustable and/or may be electromechanically adjustable. For example, a burner system may further include a controller 622 (possibly included in controller 230 of FIG. 2) having outputs configured to adjust at least one of the first valve 619 and the second valve 620. The controller 622 may be configured to cause a flow rate of the second flow of the fuel 614 to be lower than a flow rate of the first flow of the fuel 106 b when the distal flame holder 102 is at the operating temperature.

The second fuel nozzle 610 and the first fuel nozzle 118 may be configured to emit the same fuel (e.g., 106 b and 614 may be the same fuel). However, the inventors contemplate the possibility that the first and second fuel nozzles 118, 610 may deliver different fuels (e.g., 106 b and 614 may be different fuels).

In some embodiments, such a burner system may further include a heating apparatus, such as heater 228 of FIG. 2, configured to heat the distal flame holder 102 to the predetermined temperature and/or a controller, such as controller 230 or 622, operatively coupled to the heating apparatus 228. The controller 230, 622 may be configured to receive an indication of the temperature of the distal flame holder 102 and to control the heating apparatus 228 based on the indication of the temperature.

In some embodiments, the heating apparatus 228 may include the at least portions of the non-reactive fluid source 110. As suggested above with specific reference to FIGS. 6 and 7, the non-reactive fluid source 110 may have multiple modes, e.g., a startup mode and an operating mode. In the startup mode, the fuel 614 may be provided to the second fuel nozzle 610 at a relatively high rate and ignited (e.g., by ignition source 630, 730) to produce a startup/preheat flame (not shown), thermal energy from the startup/preheat flame may be received by the distal flame holder 102, heating it to the predetermined temperature.

More specifically, with respect to the heating apparatus 228 discussed with respect to FIG. 2, in some embodiments, the heating apparatus 228 may include at least the second fuel nozzle 610 configured to direct a second flow of the fuel 614 for mixture and combustion with a portion of the oxidant 106 a in the dilution region 510, the combustion of the second flow of the fuel 614 and the oxidant 106 a may provide thermal energy that heats the distal flame holder 102.

The burner system 600, 700 may further include an oxidant supply controller (e.g., as part of controller 230, 622) configured to control a flow rate of the oxidant 106 a. The oxidant supply controller 622 may include a first oxidant flow control mechanism 624 configured to control flow of the oxidant 106 a for mixture with the fuel 106 b, 614 alternately at the first fuel flow rate and the second fuel flow rate and/or a second oxidant flow control mechanism (not shown separately) configured to control flow of the oxidant 106 a for mixture of the oxidant 106 a with the fuel 106 b from the first fuel nozzle 118.

The oxidant supply controller (part of controller 230, 622) may be further configured to control flow and/or direction of a recirculated flue gas in concert with an external flue gas recirculation element. For example, the oxidant supply controller may control a damper or blower in a recirculated flue gas conduit (not shown) through which the flue gas flows. In another example, the oxidant supply controller may control a shutter or movable duct configured to adjust a direction of output of the recirculated flue gas.

A method 800 for inhibiting flashback in a burner system is illustrated in FIGS. 8A, 8B. In step 810, an oxidant (e.g., 106 a) is supplied to a combustion volume (e.g., 204). A fuel (e.g., 106 b) is directed, in step 820, via a first fuel nozzle (e.g., 118) to a dilution region (e.g., 510) of the combustion volume between the first fuel nozzle and a distal flame holder (e.g., 102). In another step 830, the oxidant is mixed with the fuel from the first fuel nozzle in the dilution region to provide a mixture (e.g., 206) of the fuel and the oxidant. The method 800 may further include, in step 840 of FIG. 8A, providing a non-reactive fluid (e.g., 116, 616) to the mixture of the fuel and the oxidant in a portion of the dilution region proximate the distal flame holder (e.g., 102).

The step 840 of providing the non-reactive fluid (e.g., 116, 616) may include, as shown in FIG. 8B, a step 842 of emitting the fuel from a second fuel nozzle (e.g., 610) at a velocity different from the fuel supplied via the first fuel nozzle. In step 844, the fuel from the second nozzle may be ignited (e.g., by the ignition source 630, 730) to combust the oxidant and fuel emitted from the second fuel nozzle in a portion of the dilution region proximate the second fuel nozzle. In step 846, the combustion reaction (e.g., flame 718) supported by the oxidant and fuel from the second fuel nozzle may be held at an intermediate flame holder (e.g., 702). The intermediate flame holder may be disposed proximate the second fuel nozzle. However, in some embodiments the inventors recognize there may be a motivation to position the intermediate flame holder a distance from the second fuel nozzle, or to provide a mechanism for relocating the intermediate flame holder between a plurality of positions. In step 848, combustion products (e.g., 616) of the flame may be provided to the fuel and oxidant mixture (e.g., 206), changing characteristics of the mixture to prevent flashback.

In some embodiments, providing the non-reactive fluid toward the distal flame holder may include emitting a non-reactive gas from a second nozzle (e.g., 110 a) into the dilution region (e.g., 510).

In some embodiments, step 848 of providing the non-reactive fluid to the mixture of the fuel and the oxidant may include recirculating a flue gas, as the non-reactive fluid, sourced from beyond the distal flame holder.

The above described embodiments focus on addressing flashback. In some of the embodiments, an intermediate flame holder (e.g., 702) is implemented. The use of an intermediate flame holder 702 in addition to a distal flame holder 102 is a novel implementation that merits the separate description below.

A multi-stage burner according to the disclosure includes two or more flame holders in succession. This differs from a conventional staged burner that introduces fuel and/or oxidant at different stages of the burner. For example, a conventional staged burner may introduce fuel, transport air, and secondary air together for combustion in a primary combustion zone. Staged fuel may be introduced for combustion at a secondary combustion (or reburning) zone and staged air may be introduced for combustion at a tertiary combustion zone. The “stages” therefore conventionally refer to stages of combustion within a particular flame as a result of introduced fuel and/or oxidant.

In contrast to a conventional staged combustion, combustion stages in the multi-stage burner of the present disclosure refer to distinct flames supported by distinct flame holders. In an example, FIG. 9 illustrates a multi-stage burner system 900 having a first or intermediate flame holder 902 and a distal flame holder 102 (e.g., as described above), each configured to simultaneously support combustion during an operation period. The first flame holder 902 may receive fuel (e.g., 614) from at least one fuel nozzle 910, whereas the distal flame holder 102 may receive fuel (e.g., 106 b) from at least one fuel nozzle 118.

In some embodiments, the multi-stage burner system 900 may include a fuel and oxidant source 202 configured to emit fuel (e.g., 106 b) and oxidant (e.g., 106 a) into a combustion volume, a distal flame holder 102 oriented to receive and ignite a first mixture (e.g., 206) of the fuel and the oxidant downstream of the fuel and oxidant source 202. At least one intermediate flame holder 902 may be disposed between the fuel and oxidant source 202 and the distal flame holder 102, and may be oriented to receive a second mixture of the fuel and the oxidant. One or more additional intermediate flame holders 902 may be disposed on a same plane between the fuel nozzle(s) 910 and the distal flame holder 102, or may be disposed serially. Disposition on a same plane may permit certain regions of the burner 900 to address local combustion inconsistencies of the distal flame holder 102, such as localized flashback phenomena. Alternatively, multiple intermediate flame holders 902 on a same plane can be used to provide a plurality of intermediate flames (e.g., 718) for preheating with corresponding plural fuel nozzles 910 configured for startup.

In some embodiments, a first set of fuel nozzles 118 and a second set of fuel nozzles 910 may each be associated in common with a same oxidant delivery structure, such as oxidant conduit 104, as illustrated in FIG. 7. In other embodiments (not shown), corresponding oxidant delivery structures may be associated with each of the first fuel nozzle(s) 118 and the second nozzle(s) 910.

In still another embodiment, each of plural subsets of the fuel nozzles 118 may respectively be associated with a corresponding oxidant delivery structure.

Serially disposed intermediate flame holders (not shown) may be used in implementations that include fuel nozzles 118 disposed at different respective distances from the distal flame holder 102.

In some embodiments, a multi-stage burner system 900 may further include an ignition source 930 disposed proximate the intermediate flame holder 902 and configured to ignite the mixture (206) of the oxidant and the fuel from the second fuel nozzle 910. The intermediate flame holder 902 may be configured to hold a flame (not shown in FIG. 9) resulting from the ignited second mixture (614) of the fuel and the oxidant.

The fuel and oxidant source 202 may in some embodiments include a fuel output configured to emit the fuel (106 b) for inclusion in at least one of the first mixture of the fuel and the oxidant (614) and the second mixture of the fuel and the oxidant (206) and/or an oxidant output configured to emit oxidant (106 a) for inclusion in the first mixture (614) of the fuel and the oxidant and the second mixture of the fuel and the oxidant (206).

The fuel output may include two or more fuel nozzles 118, 910 each in fluid connection with a fuel supply, e.g., via a fuel supply line 108.

In some embodiments, at least one of the two or more fuel nozzles (e.g., fuel nozzle 910) is oriented to direct a portion of fuel toward the at least one intermediate flame holder 902, and a remaining one or more fuel nozzles (e.g., fuel nozzle(s) 118) are oriented to direct fuel toward the distal flame holder 102.

A multi-stage burner system 900 may further include a controller 922 configured to control a rate of the fuel (106 b) directed by the fuel nozzle(s) 910 toward the intermediate flame holder 902. The controller 922 may control a rate of fuel flow by controlling operation of valves 919, 920. In some embodiments the controller 922 may control flow to the fuel nozzle 910, supplying fuel to the intermediate flame holder 902 independently from control of fuel supply to the fuel nozzle(s) 118, supplying fuel to the distal flame holder 102. In some embodiments, the controller 922 may alternatively or additionally control a flow of oxidant through the oxidant conduit 104 by controlling operation of a damper or blower, 924.

In some embodiments, the intermediate flame holder 902 may be oriented to direct thermal energy released by combustion of the fuel (106 b) and the oxidant (106 a) at the intermediate flame holder 902 toward the distal flame holder 102 to heat the distal flame holder 102 to a predetermined temperature.

In some embodiments, the intermediate flame holder 902 may be oriented to direct combustion products released by combustion of the fuel (106 b) and oxidant (106 a) at the intermediate flame holder 902 as a non-reactive fluid for dilution of the first mixture of the fuel and the oxidant (206) in a region (e.g., dilution region 510) between the intermediate flame holder 902 and the distal flame holder 102.

In some embodiments, the intermediate flame holder 902 may include an electrode 950 configured to provide an electrical charge to the second mixture of the fuel (106 b) and the oxidant (106 a). The electrical charge may control a flame conformance characteristic. In some embodiments (not shown), an electrical charge produced by the electrode 950 may constitute the intermediate flame holder 902.

FIG. 10A shows a method 1000 for utilizing a multi-stage burner system according to the disclosure. The method may include a step 1010 of directing an oxidant (e.g., 106 a) into a combustion volume (e.g., 204), and step 1020 of directing a fuel (e.g., 106 b) via a first fuel nozzle (e.g., 910) toward an intermediate flame holder (e.g., 902) disposed in a dilution region (e.g., 510) between the first fuel nozzle (910) and a distal flame holder (e.g., 102). Step 1030 provides holding a flame at the intermediate flame holder (e.g., 902) supported by a mixture of the oxidant and the fuel (e.g., 206) from the first fuel nozzle. If the distal flame holder (102) is at or above a predetermined temperature T_(P), (step 1040), the fuel is directed (step 1050) via a second fuel nozzle (e.g., 118) toward the distal flame holder 102 through the dilution region (e.g., 510). In step 1060, the oxidant is mixed with the fuel from the second fuel nozzle to provide a second mixture of the fuel and the oxidant for combustion at the distal flame holder (102). At step 1070, the second mixture of the fuel and the oxidant is diluted with substantially non-reactive combustion products (e.g., 616) of the intermediate flame or within the dilution region. The substantially non-reactive combustion products (e.g., 616) may be released by the intermediate flame as a non-reactive fluid for dilution of the mixture of the oxidant and the fuel from the first fuel nozzle in the dilution region. At step 1080, the diluted second mixture of the fuel and the oxidant are burned substantially at the distal flame holder 102.

Certain elements of the method 1000 of FIG. 10A are presented in further detail in FIG. 10B. For example, the step 1020 of directing the fuel via the first fuel nozzle may include, at step 1022, emitting the fuel from the first fuel nozzle (e.g., 910) at a first fuel flow rate during a startup period in which the distal flame holder (102) is heated by the flame to the predetermined temperature. At step 2024, when the distal flame holder (102) is at the predetermined temperature the method 1000 may include emitting the fuel from the first fuel nozzle at a second fuel flow rate. In some embodiments, the second fuel flow rate may be lower than the first fuel flow rate. Directing the fuel via the first fuel nozzle may include controlling a rate of the fuel directed toward the intermediate flame holder (e.g., 902).

The method 1000 may further include a step (not shown) of igniting, proximate the intermediate flame holder, the mixture of the oxidant and the fuel from the first fuel nozzle to produce the flame. Igniting the mixture of the oxidant and the fuel from the first fuel nozzle (e.g., 910) includes providing and using an ignition source (e.g., 930) proximate the intermediate flame holder (e.g., 902), where the ignition source is configured to ignite the mixture of the oxidant and the fuel from the first fuel nozzle.

The method 1000 may further include directing thermal energy from the flame toward the distal flame holder to heat the distal flame holder to the predetermined temperature.

In some embodiments, the intermediate flame holder may include one or more electrodes configured to produce an electrical charge proximate to and across the intermediate flame holder.

FIG. 11A is a simplified perspective view of a combustion system 1100, including another alternative distal flame holder 102, according to an embodiment. The distal flame holder 102 may include a reticulated ceramic distal flame holder, according to an embodiment. FIG. 11B is a simplified side sectional diagram of a portion of the reticulated ceramic distal flame holder 102 of FIG. 11A, according to an embodiment. The distal flame holder 102 having reticulated ceramic, of FIGS. 11A, 11B, can be implemented in the various combustion systems described herein, according to an embodiment. The distal flame holder 102 may be configured to support a combustion reaction (e.g., combustion reaction 302 of FIG. 3) of the fuel and oxidant mixture 206 received from the fuel and oxidant source 202 at least partially within (e.g., amongst the fiber of the reticulated ceramic distal flame holder 102. According to an embodiment, such distal flame holder 102 can be configured to support a combustion reaction of the fuel and oxidant mixture 206 upstream, downstream, within, and adjacent to the reticulated ceramic distal flame holder 102.

According to an embodiment, the distal flame holder body 208 can include reticulated fibers 1139. The reticulated fibers 1139 can define branching perforations 210 that weave around and through the reticulated fibers 1139. According to an embodiment, the perforations 210 are formed as passages between the reticulated fibers 1139.

According to an embodiment, the reticulated fibers 1139 are formed as a reticulated ceramic foam. According to an embodiment, the reticulated fibers 1139 are formed using a reticulated polymer foam as a template. According to an embodiment, the reticulated fibers 1139 can include alumina silicate.

According to an embodiment, the reticulated fibers 1139 can be formed from extruded mullite or cordierite. According to an embodiment, the reticulated fibers 1139 can include Zirconia. According to an embodiment, the reticulated fibers 1139 can include silicon carbide.

The term “reticulated fibers” refers to a netlike structure. According to an embodiment, the reticulated fibers 1139 are formed from an extruded ceramic material. In reticulated fiber embodiments, the interaction between the fuel and oxidant mixture 206, the combustion reaction, and heat transfer to and from the distal flame holder body 208 can function similarly to the embodiment shown and described above with respect to FIGS. 2-4. One difference in activity is a mixing between perforations 210, because the reticulated fibers 1139 form a discontinuous distal flame holder body 208 that allows flow back and forth between neighboring perforations 210.

According to an embodiment, the network of reticulated fibers 1139 is sufficiently open for downstream reticulated fibers 1139 to emit radiation for receipt by upstream reticulated fibers 1139 for the purpose of heating the upstream reticulated fibers 1139 sufficiently to maintain combustion of a fuel and oxidant mixture 206. Compared to a continuous distal flame holder body 208, heat conduction paths (such as heat conduction paths 312 in FIG. 3) between reticulated fibers 1139 are reduced due to separation of the reticulated fibers 1139. This may cause relatively more heat to be transferred from a heat-receiving region or area (such as heat receiving region 306 in FIG. 3) to a heat-output region or area (such as heat output region 310 of FIG. 3) of the reticulated fibers 1139 via thermal radiation (shown as element 304 in FIG. 3).

According to an embodiment, individual perforations 210 may extend between an input face 212 to an output face 214 of the distal flame holder 102.

The perforations 210 may have varying lengths L. According to an embodiment, because the perforations 210 branch into and out of each other, individual perforations 210 are not clearly defined by a length L.

According to an embodiment, the distal flame holder 102 is configured to support or hold a combustion reaction (see element 302 of FIG. 3) or a flame at least partially between the input face 212 and the output face 214. According to an embodiment, the input face 212 corresponds to a surface of the distal flame holder 102 proximal to the fuel nozzle 218 or to a surface that first receives fuel. According to an embodiment, the input face 212 corresponds to an extent of the reticulated fibers 1139 proximal to the fuel nozzle 218. According to an embodiment, the output face 214 corresponds to a surface distal to the fuel nozzle 218 or opposite the input face 212. According to an embodiment, the input face 212 corresponds to an extent of the reticulated fibers 1139 distal to the fuel nozzle 218 or opposite to the input face 212.

According to an embodiment, the formation of thermal boundary layers 314, transfer of heat between the distal flame holder body 208 and the gases flowing through the perforations 210, a characteristic perforation width dimension D, and the length L can each be regarded as related to an average or overall path through the distal flame holder 102. In other words, the dimension D can be determined as a root-mean-square of individual Dn values determined at each point along a flow path. Similarly, the length L can be a length that includes length contributed by tortuosity of the flow path, which may be somewhat longer than a straight line distance TRH from the input face 212 to the output face 214 through the distal flame holder 102. According to an embodiment, the void fraction (expressed as (total distal flame holder 102 volume−reticulated fiber 1139 volume)/total volume)) is about 70%.

According to an embodiment, the reticulated ceramic distal flame holder 102 is a tile about 1″×4″×4″. According to an embodiment, the reticulated ceramic distal flame holder 102 includes about 10 pores per square inch, which is to say that line laid across the surface of the tile would cross about 10 pores per inch of line. Other materials and dimensions can also be used for a reticulated ceramic distal flame holder 102 in accordance with principles of the present disclosure.

According to an embodiment, the reticulated ceramic distal flame holder 102 can include shapes and dimensions other than those described herein. For example, the distal flame holder 102 can include reticulated ceramic tiles that are larger or smaller than the dimensions set forth above. Additionally, the reticulated ceramic distal flame holder 102 can include shapes other than generally cuboid shapes.

According to an embodiment, the reticulated ceramic distal flame holder 102 can include multiple reticulated ceramic tiles. The multiple reticulated ceramic tiles can be joined together such that each ceramic tile is in direct contact with one or more adjacent reticulated ceramic tiles. The multiple reticulated ceramic tiles can collectively form a single distal flame holder 102. Alternatively, each reticulated ceramic tile can be considered a distinct distal flame holder 102.

Turning now to FIG. 12, a burner system 1200 may include a distal flame holder 102, a plurality of main fuel nozzles 118, one or more distal pilot fuel nozzles 1204, and a mixing tube 1212. The main fuel nozzles 118 are arranged in fluid connection with a main fuel source 1232. According to an embodiment, flow of main fuel from the main fuel source 1232 may be controlled via a main fuel control valve 1236. The one or more distal pilot fuel nozzles 1204 may be arranged in fluid connection with a pilot fuel source 1230. According to an embodiment, flow of pilot fuel from the pilot fuel source 1230 may be controlled via a pilot fuel control valve 1234.

The pilot fuel nozzle 1204 is configured to support a pilot flame by outputting a pilot fuel received via a pilot fuel pipe 1210 from the pilot fuel source 1230. The pilot fuel pipe 1210 may be disposed inside the mixing tube 1212 or—advantageously for maintenance, temperature regulation, etc.—outside the mixing tube 1212. In some embodiments, the pilot fuel pipe 1210 may form a portion of a support for the mixing tube 1212. According to an embodiment, the pilot fuel nozzle 1204 is supported by and receives fuel via the fuel pipe 1220.

The fuel pipe 1220 extends into the furnace volume 1201 via the opening 1240 in the floor 1238 of the furnace. The pilot fuel nozzle 1204 may be formed in any of several shapes. For example, in FIG. 12, the pilot fuel nozzle 1204 is formed in a Y shape

According to an embodiment, the pilot fuel nozzle 1204 defines a plurality of fuel orifices 1218 having a large collective area to collectively support a low momentum pilot flame (not shown). In an embodiment, the main fuel output by the main fuel nozzles 118 and combustion air form a combustible mixture that expands in breadth as it flows from a proximal position of the main fuel nozzles 118 to the distal position of the pilot fuel nozzle 1204. The plurality of fuel orifices 1218 may be disposed across the furnace volume 1201 sufficiently broadly to cause contact of the pilot flame with the main fuel and combustion air mixture across the breadth of the combustible mixture. In another embodiment, the main fuel nozzles 118 may be configured to output fuel in co-flow with the air.

According to an embodiment, the primary fuel nozzle 1204 includes a fuel manifold having a plurality of segments 1219 joined together, each segment 1219 having a plurality of fuel orifices 1218 configured to pass fuel from inside the fuel manifold to the furnace volume 1101. The plurality of segments 1219 may be formed as respective tubes configured to freely pass the fuel delivered from the fuel pipe 1210 into the fuel manifold. In one embodiment (e.g., as in FIG. 12), at least a portion of the tubes is arranged as spokes radiating from a center disposed substantially at a centerline along the axis. In another embodiment, at least a portion of the tubes is arranged as an “X”, a rectangle, an “H”, a wagon wheel, or a star.

According to an embodiment, the pilot fuel nozzle 1204 includes a manifold including a curvilinear tube. In an embodiment, the curvilinear tube is arranged as a spiral, “

”, “

”, or “

”.

According to an embodiment, the mixing tube 1212 may be arranged about a longitudinal axis of flow between the main fuel nozzles 118 and the distal flame holder 102. According to an embodiment, the mixing tube 1212 may include a bell-shaped or flared portion 1214 at an end proximate the main fuel nozzles 118. The bell-shaped or flared portion 1214 may be disposed a predetermined distance from a floor 1238 of the burner system, and may be configured to receive at least the combustion air via an opening 1240 in the floor 1238. As described earlier in this disclosure, a source of non-reactive fluid in contemplated. The inventors have observed that the introduction of a mixing tube facilitates a recirculation of flue gas—as a substantially non-reactive fluid—from downstream of the distal flame holder 102, and/or including combustion products of a pilot flame held at the pilot fuel nozzle. The flue gas is educed to a proximal end of the mixing tube 1212 by a flow of main fuel and combustion oxidant between the floor 1238 and the distal flame holder 102 through the mixing tube 1212. The recirculated flue gas, and mixes with the fuel and the combustion air before reaching the distal flame holder 102. The non-reactive elements of the resulting mixture minimize a potential for flashback upstream from the distal flame holder 102 while permitting additional combustion of the reactive elements of the flue gas, thus reducing, e.g., NOx and other potential pollutants.

Those having skill in the art will recognize the FIG. 12 should not be relied upon as representing appropriate scale, relative dimensions, shapes, etc. For example, the mixing tube 1212 may have a diameter appropriate for providing a mixture of fuel and oxidant (e.g., fuel and oxidant mixture 206) to at least most of the input face (e.g., input face 212) of the distal flame holder 102. The opening at the end of the mixing tube 1212 closest to the main fuel nozzles 118 may have a largest diameter sized in correspondence to either the opening 1240 in the floor 1238 or sufficient to receive fuel input from each of the main fuel nozzles 118. For example, in an embodiment that includes the bell-shaped or flared portion 1214, the largest diameter of the bell-shaped or flared portion 1214 may correspond to either the opening 1240 in the floor 1238 or may correspond to at least the farthest distance between main fuel nozzles 118. A length of the mixing tube may be selected to permit sufficient time and/or distance for appropriate mixing of the fuel and the oxidant before reaching the distal flame holder 102.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 

1. A burner system, comprising: a distal flame holder for disposition in a combustion volume and configured to receive and ignite a fuel and oxidant mixture; a fuel source configured to contribute a fuel to the fuel and oxidant mixture; an oxidant source configured to contribute an oxidant to the fuel and oxidant mixture; and a non-reactive fluid source configured to deliver a non-reactive fluid in a dilution region between the distal flame holder and the non-reactive fluid source.
 2. The burner system of claim 1, further comprising: a mixing tube disposed in the dilution region and being open between the distal flame holder and the fuel source, the mixing tube configured to receive, and facilitate mixing of, the fuel and the oxidant; and a pilot fuel nozzle disposed proximate the distal flame holder and downstream from a distal end of the mixing tube.
 3. The burner system of claim 2, wherein the non-reactive fluid is flue gas from downstream of the distal flame holder, and a flow of the fuel and oxidant from the fuel and oxidant source toward the distal flame holder draws the flue gas into a proximal end of the mixing tube from at least downstream of the distal flame holder along an outer surface of the mixing tube.
 4. The burner system of claim 2, wherein the proximal end of the mixing tube includes a flared portion.
 5. The burner system of claim 1, further comprising a pilot flame holding feature disposed to hold a pilot flame; wherein the non-reactive fluid source is configured to deliver the non-reactive fluid in a region disposed between the pilot flame holding feature and an ignitable portion of the fuel and oxidant mixture.
 6. The burner system of claim 5, wherein the non-reactive fluid includes a reactive component; and wherein the pilot flame held by the pilot flame holding feature generates at least a portion of the non-reactive fluid in the form of a fuel depleting combustion product.
 7. The burner system of claim 6, wherein the non-reactive fluid includes one or more components selected from the group consisting of: nitrogen, carbon dioxide, nitrogen oxide, nitrogen dioxide, sulfur oxide, sulfur dioxide, oxygen, and argon.
 8. The burner system of claim 7, further comprising: an ignitor configured to selectably ignite the fuel and oxidant mixture, the ignitor being disposed adjacent to the pilot flame and the fuel and oxidant mixture.
 9. The burner system of claim 3, wherein the non-reactive fluid further comprises combustion reaction products generated by a pilot flame.
 10. The burner system of claim 1, wherein the fuel source includes: a fuel supply configured to store the fuel, and a first fuel nozzle in fluid connection with the fuel supply and positioned to deliver the fuel to the fuel and oxidant mixture; and the non-reactive fluid source includes: a second fuel nozzle in fluid connection with the fuel supply, an intermediate flame holder disposed between the second fuel nozzle and the distal flame holder, the intermediate flame holder configured to hold a flame supported by the oxidant and by the fuel, when the fuel is delivered from the second fuel nozzle, and an ignition source configured to ignite the fuel and the oxidant at the intermediate flame holder to produce the flame.
 11. The burner system of claim 10, wherein combustion products from the flame constitute at least a portion of the non-reactive fluid.
 12. The burner system of claim 11, further comprising: a fuel flow control mechanism in fluid connection with the second fuel nozzle; wherein the fuel flow control mechanism is configured to permit a first fuel flow rate to the second fuel nozzle during a startup period and is configured to permit a second fuel flow rate to the second fuel nozzle after the startup period.
 13. The burner system of claim 12, wherein the intermediate flame holder is disposed a distance from the distal flame holder sufficient at the first fuel flow rate to cool the combustion products below an auto-ignition temperature of the fuel and oxidant mixture. 14.-15. (canceled)
 16. The burner system of claim 1, wherein the distal flame holder is configured to support a combustion reaction of the fuel and the oxidant upstream, downstream, and within the distal flame holder. 17.-23. (canceled)
 24. A burner system, comprising: a distal flame holder configured to hold a combustion reaction of a fuel and an oxidant; an oxidant conduit configured to direct the oxidant toward the distal flame holder; a first fuel nozzle oriented to direct a first flow of the fuel into a combustion volume for mixture with the oxidant in a dilution region between the first fuel nozzle and the distal flame holder when a temperature of the distal flame holder is above a predetermined temperature; a mixing tube disposed in the dilution region, and being open between the distal flame holder and the first fuel nozzle; and a non-reactive fluid source oriented to provide a non-reactive fluid into the mixing tube when the distal flame holder is at an operating temperature.
 25. The burner system of claim 24, wherein the non-reactive fluid source comprises: a flue gas recirculation path; wherein flue gas provided by via flue gas recirculation path constitutes at least part of the non-reactive fluid, and the flue gas is educed into a fuel and oxidant stream by the flow of at least the oxidant.
 26. The burner system of claim 25, further comprising; a pilot fuel nozzle configured to provide pilot fuel for a pilot flame, wherein the flue gas recirculation path includes at least a toroidal volume between the mixing tube and a wall of the combustion volume, and the flue gas is educed into the fuel and oxidant stream at a proximal opening of the mixing tube for dilution of the fuel and oxidant stream.
 27. The burner system of claim 26, wherein the flue gas recirculation path is external to combustion chamber.
 28. The burner system of claim 27, further comprising a flue gas flow control mechanism configured to control at least one of a rate of flow and a direction of flow of the recirculated flue gas.
 29. The burner system of claim 24, wherein the mixing tube includes a flared portion at the proximal opening.
 30. The burner system of claim 24, further comprising; a pilot fuel nozzle disposed lateral to the first fuel nozzle; and an ignition source disposed proximate the pilot fuel nozzle and configured to ignite a mixture of the oxidant and the fuel emitted from the pilot fuel nozzle.
 31. The burner system of claim 30, further comprising: a first valve configured to control a flow rate of the first flow of the fuel to the first fuel nozzle; and a second valve configured to control a flow rate of a second flow of the fuel to the pilot fuel nozzle.
 32. The burner system of claim 31, further comprising: a controller having outputs configured to adjust at least one of the first valve and the second valve, wherein the controller is configured to cause the flow rate of the second flow of the fuel to be lower than the flow rate of the first flow of the fuel when the distal flame holder is at the operating temperature.
 33. The burner system of claim 30, wherein the pilot fuel nozzle and the first fuel nozzle are configured to emit the same fuel.
 34. The burner system of claim 24, further comprising: a heating apparatus configured to heat the distal flame holder to the predetermined temperature; and a controller operatively coupled to the heating apparatus, the controller configured to receive an indication of the temperature of the distal flame holder and to control the heating apparatus based on the indication of the temperature.
 35. The burner system of claim 34, wherein the heating apparatus includes the non-reactive fluid source, and the non-reactive fluid source includes at least the pilot fuel nozzle configured to direct the second flow of the fuel for mixture and combustion with a portion of the oxidant in the dilution region, the combustion of the second flow of the fuel and the oxidant providing thermal energy that heats the distal flame holder, and wherein the controller is configured to cause the pilot fuel nozzle to emit the second flow of the fuel at a first fuel flow rate when the temperature of the distal flame holder is below the predetermined temperature, and to emit the second flow of the fuel at a second fuel flow rate when the temperature of the distal flame holder is at or above the predetermined temperature.
 36. The burner system of claim 35, wherein the first fuel flow rate is higher than the second fuel flow rate.
 37. The burner system of claim 35, further comprising an oxidant supply controller configured to control a flow rate of the oxidant, the oxidant supply controller including: a first oxidant flow control mechanism configured to control flow of the oxidant for mixture with the fuel alternately at the first fuel flow rate and the second fuel flow rate; and a second oxidant flow control mechanism configured to control flow of the oxidant for mixture of the oxidant with the fuel from the first fuel nozzle.
 38. (canceled)
 39. A method for inhibiting flashback in a burner system, the method comprising: supplying an oxidant to a combustion volume; directing a fuel via a first fuel nozzle to a dilution region of the combustion volume between the first fuel nozzle and a distal flame holder; mixing the oxidant with the fuel from the first fuel nozzle in the dilution region to provide a mixture of the fuel and the oxidant; and providing a non-reactive fluid to the mixture of the fuel and the oxidant in the dilution region.
 40. The method of claim 39, wherein said providing the non-reactive fluid includes: emitting the fuel from a second fuel nozzle at a velocity different from a velocity of the fuel supplied via the first fuel nozzle; and combusting the fuel emitted from the second fuel nozzle in a portion of the dilution region proximate the second fuel nozzle, wherein combustion products from combusting the fuel emitted from the second fuel nozzle constitute the non-reactive fluid.
 41. The method of claim 40, wherein said combusting the fuel emitted from the second fuel nozzle in a portion of the dilution region proximate the second fuel nozzle comprises: supplying the oxidant proximate the second fuel nozzle toward an intermediate flame holder disposed in the dilution region; igniting a second mixture of the fuel emitted from the second fuel nozzle and the oxidant supplied proximate the second fuel nozzle; and holding a flame, resulting from the ignition of the second mixture, at the intermediate flame holder.
 42. The method of claim 41, wherein the intermediate flame holder is disposed proximate the second fuel nozzle.
 43. (canceled)
 44. The method of claim 39, wherein said providing the non-reactive fluid to the mixture of the fuel and the oxidant includes recirculating a flue gas, as the non-reactive fluid, sourced from downstream of the distal flame holder.
 45. The method of claim 44, further comprising: educing the flue gas into a proximal end of a mixing tube, the mixing tube disposed in the dilution region and open between the first fuel nozzle and the distal flame holder, for dilution of the mixture of the fuel and the oxidant. 46.-53. (canceled)
 54. The multi-stage burner system of claim 10, wherein the intermediate flame holder comprises an electrode configured to provide an electrical charge to the second mixture of the fuel and the oxidant.
 55. A method of utilizing a multi-stage burner system, the method comprising: directing an oxidant into a combustion volume; directing a fuel via a first fuel nozzle toward an intermediate flame holder disposed in a dilution region between the first fuel nozzle and a distal flame holder; holding a flame at the intermediate flame holder supported by a mixture of the oxidant and the fuel from the first fuel nozzle; and when the distal flame holder is at a predetermined temperature: directing the fuel via a second fuel nozzle toward the distal flame holder via the dilution region, mixing the oxidant and the fuel from the second fuel nozzle to provide a second mixture of the fuel and the oxidant for combustion at the distal flame holder, diluting the second mixture of the fuel and the oxidant with combustion products of the flame within the dilution region, and burning the second mixture of the fuel and the oxidant proximate to at the distal flame holder.
 56. The method of claim 55, wherein said directing the fuel via the first fuel nozzle includes: emitting the fuel from the first fuel nozzle at a first fuel flow rate during a startup period in which the distal flame holder is heated by the flame to the predetermined temperature; and emitting the fuel from the first fuel nozzle at a second fuel flow rate when the distal flame holder is at the predetermined temperature, the second fuel flow rate being lower than the first fuel flow rate.
 57. The method of claim 55, further comprising igniting, proximate the intermediate flame holder, the mixture of the oxidant and the fuel from the first fuel nozzle to produce the flame.
 58. The method of claim 57, wherein said igniting the mixture of the oxidant and the fuel from the first fuel nozzle includes providing an ignition source proximate the intermediate flame holder, the ignition source configured to ignite the mixture of the oxidant and the fuel from the first fuel nozzle.
 59. The method of claim 55, wherein said directing the fuel via the first fuel nozzle includes controlling a rate of the fuel toward the intermediate flame holder.
 60. The method of claim 55, further comprising: directing thermal energy from the flame toward the distal flame holder to heat the distal flame holder to the predetermined temperature.
 61. The method of claim 55, further comprising: directing the combustion products released by the flame as a non-reactive fluid for dilution of the mixture of the oxidant and the fuel from the first second fuel nozzle in the dilution region.
 62. The method of claim 55, further comprising powering an electrode to produce an electric charge across the intermediate flame holder. 